<!DOCTYPE html>
<html>
<head>
    <meta charset="utf-8">
    <title></title>
    <style>
        .container {
            display: flex;
            gap: 1rem;
            padding: 1rem;
            background-color: red;
            color: white;
        }

        .box {
            float: left;
            background-color: blue;
            padding: 1rem;
        }

        .fx {
            width: 300px;
        }

        .fill {
            flex: auto;
        }

    </style>
</head>
<body>
<div class="container">
    <div class="box fx">Fixed</div>
    <div class="box fx">Fixed</div>
    <div class="box fx">Fixed</div>
    <div class="box fill">Fill</div>
</div>
</body>
</html>